1. Field of the Invention
The present invention relates to a method and an apparatus for correcting a color imbalance of visible light in wavelength division parallel visible light communications. More particularly, the present invention relates to a method and an apparatus for generating a visible light signal which corrects the energy differences among wavelengths when parallel transmission is implemented by using multiple wavelengths different from one another in a visible light communication (VLC) system.
2. Description of the Related Art
Recently, Light Emitting Diodes (LEDs) have been improved in luminous efficiency thereof, and have also dropped in unit price thereof. Accordingly, LEDs are now more commonly used not only in a special illumination market, such as handheld devices, displays, automobiles, traffic lights, advertising boards, etc., but also in a general illumination market, such as fluorescent lamps, incandescent electric lamps, etc. Also, as the interest in optical wireless technology complementary with RF technology has increased due to an exhaustion of the frequencies in a Radio Frequency (RF) band, and the possibility of a false cross among wireless communications coupled with an increase in the security requirement for communications, the advent of a very high-speed ubiquitous communication environment of a fourth generation mobile communication (4G) wireless technology, etc., is being studied. In particular, the use of visible light wireless communications using visible light LEDs is being studied by many enterprises and research institutes, etc.
Visible light communications, which transmits information by using light in the visible spectrum, provides some advantages that include a wide use band, and the ability to be freely used without being subject to regulation to the extent that other forms of communication are regulated. Also, visible light communications has an advantage in that the reception range of information can be accurately sensed because a spot where light reaches or a direction in which the light moves can be seen by a user. Accordingly, visible light communications have reliability with regard to security, and also have merit such as the ability to be drive with lower electric power than some of the other forms of communication.
Luminous elements for visible light communications have made rapid progress in recent days, but such luminous elements are not able to turn on/off at high speed. For example, in a case of a white LED using phosphor, its manufacturing cost is relative inexpensive but its modulation speed is no more than about 10 [Mbps]. In order to overcome this relatively slow modulation speed, studies are proceeding on a scheme in which visible light having information is generated from each LED by using multiple LEDs for generating three primary colors, including Red, Green, and Blue (RGB), and the generated visible lights are mixed to make white light. A scheme of transmitting signals in parallel by using the multiple LEDs for generating the three primary colors has an advantage in that high-speed transmission can be implemented, but in a case where respective energy distributions of wavelengths are different from one another, there appears a problem such that generated light can have a color tone other than white. If the generated light corresponds to light having any color tone other than the white light, this becomes a serious disadvantage in the visible light communications in which a transmitter serves as a lighting device at the same time. Hereinafter, a description will be made of a general apparatus for wavelength division parallel visible light communications, which transmits signals in parallel by using the multiple LEDs.
FIG. 1 is a block configuration diagram illustrating an example of a general transceiver for wavelength division parallel visible light communications. With reference to FIG. 1, a transmitter 101 for visible light communications includes multiple encoders 105, multiple modulators 111, a light generator (not shown), and a controller 103. Herein, the multiple encoders 105 are configured in parallel, and perform channel coding on data to be transmitted, respectively. The multiple modulators 111 are configured in parallel, and modulate respective channel-coded data from one of the multiple encoders 105. The light generator (not shown) transmits signals modulated by the multiple modulators 111 as visible signals, respectively. The controller 103 controls each configuration element of the transmitter 101 for visible light communications.
A receiver 102 for visible light communications includes a light sensor (not shown), multiple demodulators 112, multiple decoders 106, and a controller 104. Herein, the light sensor (not shown) receives visible light signals. The multiple demodulators 112 are configured in order to demodulate the visible light signals received by the light sensor, (not shown) respectively. The multiple decoders 106 receive respective signals demodulated by the multiple demodulators 112, perform channel decoding on the respective received signals in order to restore the respective received signals to original states thereof, and provide data. The controller 104 controls configuration elements of the receiver 102 for visible light communications. In the apparatus for wavelength division parallel visible light communications, paths independently operate by determining each path after through which path information is to be transmitted.
FIG. 2 is a flowchart illustrating transmission/receive (Tx/Rx) operations of wavelength division parallel visible light communications in general. Referring to FIG. 2, the transceiver for visible light communications begins to operate at step 200. In step 210, the transmitter for visible light communications transmits a visible light signal, and in step 220, the receiver for visible light receives the visible light signal from the transmitter for visible light communications. Thereafter, in step 230, it is determined whether the Tx/Rx operations are completed. If it is determined that the Tx/Rx operations are completed, in step 240 the method ends. However, if the operations are not complete at step 230, the procedure returns back to step 210, and the Tx/Rx operations are repeatedly performed it is determined that the Tx/Rx operations have been completed.
In the meantime, a color balance refers to a state where a final mixing of light has white color, as energy distributions in the specific ratio are achieved by the combination of each of wavelengths of visible light. A mixture of light is determined according to the energy ratio among the three primary colors. Namely, the energy distribution rate according to wavelength of light determines a color tone of the light. Specially, the white light has electric power existing over all the wavelength bands. The relation between energy by wavelengths and a color tone of the light can be found with reference to a chromaticity diagram.
Since parallel transmission can be achieved if different information is transmitted by each wavelength with dividing wavelengths of light by lengths, high-speed luminous elements are not required. The receiver filters a received light signal through an optical filter, and can extract and recover only from a signal having a desired wavelength. Because different information is transmitted by each wavelength, if the scheme of parallel transmission using the wavelength division is viewed from the aspect of the color balance, an energy balance may not be kept among multiple wavelengths, and visible light generated from the loss of the energy balance cannot provide white light. If the generated light corresponds to light having a color tone other than the color white, this difference in color tone becomes a serious problem in the visible light communications in which the transmitter serves as a lighting device at the same time engaging in communications.